Extensive work is being carried out in the development of materials suitable for optical waveguiding devices. Principal problems in the fabrication of optical waveguiding devices are the need for high optical quality including freedom from defects and smooth change from substrate to optical film. It is also desirable for many applications to have a sharp and large change in refractive index from substrate to optical film. This produces widely separated modes which are easy to couple into and out of integrated optical circuits.
Particularly attractive, principally because of high index of refraction, is thin films of LiNbO.sub.3. Growth techniques reported in the literature include diffusion, vapor deposition and liquid phase epitaxial processes. The compound LiTaO.sub.3 is commonly used as the substrate because it is isostructural with LiNbO.sub.3 which permits epitaxial growth of thin films. None of the earlier methods are, however, completely satisfactory. For example, diffusion process produces a layer with relatively small refractive index difference between film and substrate (usually less than 1%), although it does have the advantage of leaving the substrate surface in its original highly polished state, which greatly facilitates device fabrication. Liquid phase epitaxial growth through flux is a useful process, but the flux usually introduces additional loss to the circuit and thus undesirable for some applications.
The melt phase process produces a large refractive index change between substrate and film but the nature of the high temperature melt phase reaction sometimes introduces optical imperfections. For example, Miyazawa (Appl. Phys. Lett. 23, 198 (1973)) describes melt phase epitaxial growth of LiNbO.sub.3 on LiTaO.sub.3 substrates. The films were grown on the C and A planes. It is highly desirable to have a process for making optical waveguiding circuits which yield both high index changes and leave the substrate surface in its original polished condition.